How to Build a Great CW Filter for the Ten Tec Triton IV

My Ten Tec Triton IV Love Affair

–>> Update, July 14, 2021: I have a few bare circuit boards for the filter described below available for sale. Buyers will provide their own parts. The boards are $20 (shipping included) and will include a parts list, schematic diagram and installation instructions for the Ten Tec Triton Series radios. Contact me through this site (jim@jimkerkhoff.com) or wb0mmc@arrl.net if you are interested. Now, back to our regularly scheduled program.

As I mentioned in an earlier post, I owned a Ten Tec Triton IV (model 544) in the late 70’s and always loved it. I traded the radio a few years later for an ICOM 720a (also a great radio), but always regretted losing the Triton. Fast forward about 40 years, and the gift of an old Triton II reminded of that first love. The Triton II is similar, but the IV had several refinements that made it really great at the time, and I decided to get one again. Fortunately, Ten Tec equipment is readily available on the auction and swap sites, and I had one in just a couple of weeks. A ham friend decided to get one as well. The two radios arrived in my shop and previous posts describe the restoration process of my friend’s 544.

I Need a CW Filter!

As luck would have it, my friend’s radio came with the optional CW filter and mine didn’t. This filter is (in my humble opinion) the heart of the 544’s greatness — especially for a Morse Code (CW) op. The early series of Ten Tec transceivers transformed the CW world with full break-in (QSK) and this four-stage, op-amp-based audio filter. Of course audio filters for CW had been around for decades, but it was Ten Tec’s insight to place the filter in the audio chain before deriving AGC voltage that made it really special. As a result, the radio’s automatic gain was controlled by the signal in the audio passband, eliminating the audio “pumping” from nearby signals. Magic. Ten Tec sold the filter (model 245) as an optional plug-in board for $19 in 1975. This filter also fits the Triton I and II.

The Genesis of the Project

Since neither the Triton IV or Triton II I owned had the mythic filter, I began looking for one. They seemed to be rare, and expensive when one occasionally popped up — typically going for $100 or more. This is a little ironic considering that complete Ten Tec radios of that era typically sell for around $300. That’s an expensive part. So what about building one? I added it to my project list, planning to create one using a perfboard and a lot of soldering. Then, while learning KiCad, a computer-aided circuit design application for another project, I realized that this would be an excellent way to reproduce the filter. KiCad produces files that are compatible with several online fabricators, and the cost of ordering professionally manufactured PC boards it very reasonable. Turnaround is fast too — usually only a week or two — including shipping time.

Updating the Filter Design

I planned to follow Ten Tec’s original design. Further research revealed something interesting though. First of all, they used well-understood op amp engineering dating back to the 70’s and designs for an active bandpass filter are readily found on the Web. Second, versions of the same 4-stage filter were used in many Ten Tec models including the Triton, Corsair, and early Omni radio lines. It seemed the same circuit board and pin out would fit several of their models.

There were differences with later models of the filter, but they are all functionally equivalent. Finally, the original model 245 filter used the µA747CN dual op amp — still available, but dated. Later, Ten Tec used the LM3900 quad Norton op amp, and even later, the RC4136 quad op amp.  Of course, there are many other choices, and having worked in the professional audio world, I thought about using the RC4558 or the LM833 dual op amps — both low cost and widely available. Both come in an 8-pin “Dual Inline Package” (DIP), and would be interchangeable if socketed.

Designing the New Circuit Board in KiCad

After deciding to work with the 8-pin DIP dual op amps, I started with KiCad. There are several other computer-aided design (CAD) applications for PC boards, but I liked the open-source and therefore low-cost nature of KiCad. They have a robust user community. Excellent documentation can be found on their site, and there are also many YouTube videos available. I found this video particularly helpful.

No doubt, KiCad is a complete system with schematic diagram layout, extensive component libraries, PC board layout, 3D visualization and Gerber file production. The video even had tips on ordering from one of the online PC board manufacturers. It took a few hours to draw the schematic, convert it to a PC board and order the boards. The ordering process was pretty easy, and I had five 2-layer PC boards in about a week. They looked great, and were drilled with plated-through holes, silkscreened and ready to solder.

Building the Filter Board

I already had the parts for one board on hand, and it took a little over an hour to assemble. The board is exactly the same size as the original filter board, with seven connector pins and three mounting holes meant to match the original. I got the pins right, but the mounting holes had to be enlarged a bit to fit the radio. Parts consist of 1/4 watt metal oxide resistors with a 5% tolerance, and 5% monolithic chip capacitors. There is one electrolytic capacitor.

I decided to use IC sockets so I could try different op amps. The first board was assembled with RC4558s. The board connector consists of gold-plated 1 mm pins, and they mate with the 7-pin socket in the radios. Most of the circuit traces are on the top side of the board, with the bottom mostly being a ground plane. This should help with preventing noise and feedback.

Testing and Installing

Once complete, I double-checked all the connections and powered the board for the first time. No smoke — a good sign. With RC4558 op amps, the board consumes about 7 mA, as compared with about 3 mA from an original Ten Tec filter. Feeding a signal from an audio sweep generator into the board, I set an oscilloscope up to plot the filter response, and sure enough, each of the four board outputs was progressively narrower. There was little distortion and no noise, even over a wide range of input signal levels. Ten Tec powered these filters with about 9-volts in some radios and 12-14 volts in others. The new board handled these power differences just fine.

Ten Tec only uses two of the available filter outputs for the Triton-series radios. Output 1 (labeled “CW-2” on the Triton front panel) is about 600 Hz wide at the half-power points. Output 4 (labeled “CW-1”) is about 300 Hz wide — similar to the Ten Tec original filter.

Finally, I installed it into my 544, and listened to CW and voice signals on the air. To my ears, it sounds a little better than the original. Ten Tec only uses two of the four outputs in the Triton IV so there are actually three audio positions: No filter for voice (SB-N and SB-R), CW 1 and CW 2. The center frequency on the filter is about 770 Hz, and CW-1 and CW-2 seem to track well. CW-2 is also useful for some voice reception, removing adjacent signals and reducing noise well. I wouldn’t want to listen to SSB that way all the time, but it’s useful for a crowded or noisy band.

Although the filter is designed to fit into Ten Tec radios, there’s no reason it couldn’t be used with other equipment. Mounted in a little box with a 9-volt battery, it could used to enhance the CW reception of classic old receivers — something I plan to do.